1. Field of the Invention
The present invention relates to a shift-by-wire type shift switching device and shift switching method that switch the shift range of an automatic transmission by an electric motor.
2. Description of the Related Art
In a vehicle powered by an engine (internal combustion engine), automatic transmissions that automatically set an optimal gear ratio between the engine and the drive wheels are conventionally used to transmit the torque and rotational speed generated by the engine to drive wheels in accordance with the running state of the vehicle.
Examples of automatic transmissions that a vehicle may be equipped with include planetary gear type transmissions that set a gear step using a clutch, a brake, and a planetary gear device, and belt type continuously variable transmissions (CVT) that adjust the gear ratio steplessly.
In vehicles equipped with a planetary gear type automatic transmission, a gear shift map having gear shift lines (gear step switching lines) for obtaining an optimum gear step according to the vehicle speed and the throttle opening (or accelerator operation amount) may be stored in an ECU (Electronic Control Unit). A target gear step is computed based on the vehicle speed and a throttle opening by referencing the gear shift map, and based on the target gear step, frictional engagement elements such as a clutch, a brake, and a one-way clutch are engaged or released into a predetermined state to thereby automatically set the gear step (gear shift step).
In addition, a belt-type continuously variable transmission is configured such that a belt is wound around a primary pulley (input-side pulley) and a secondary pulley (output-side pulley) each having a pulley groove (V groove), and the groove width of the pulley groove of one of the pulleys is increased while reducing the groove width of the pulley groove of the other pulley, thereby continuously varying the winding radii (effective diameters) of the belt on the respective pulleys to thereby set the gear ratio steplessly.
As an example of a control device for controlling such an automatic transmission, there is a shift switching device of a so-called shift-by-wire type, which electrically detects the shift range position of the automatic transmission by a sensor, and switches the manual valve of the automatic transmission by driving a shift-switching electric motor based on the detection signal, thereby switching shift positions such as P (Parking), R (Reverse), N (Neutral), and D (Drive).
According to such a shift-by-wire type shift switching device, unlike a common shift switching device, that is, unlike a shift switching device of a type in which the shift range of the automatic transmission is directly switched by a driver's shift lever operation, there is no need for a mechanical connection between the shift lever and the shift range switching mechanism. Thus, there are no constraints on layout when installing these components in the vehicle, thus achieving enhanced freedom of design. Also, the shift-by-wire type shift switching device has the advantage of allowing easy mounting onto the vehicle.
As an example of an electric motor applied to a shift switching device, there is a brushless type motor such as a switched reluctance motor (SR motor) which is structurally simple and inexpensive. Also, some of motors applied to a shift switching device are of a type which is installed with an encoder that outputs a pulse signal synchronously with the rotation of a rotor, counts the pulse signal of the encoder, and detects the rotational position of the rotor based on the count value to sequentially switch a current supply phase, thereby rotationally driving the rotor. In this type of a motor with an encoder, since the rotational position of the rotor may be detected based on the encoder count value after start-up, it is possible to rotate the rotor to a target position through a feedback control system (F/B control system).
In the case of a shift-by-wire type shift switching device, there is a fear that switching of the shift range may become impossible in the event of a fault in a shift-switching electric motor. Thus, it is necessary to carry out a failsafe control Examples of techniques related to a motor fault include the techniques described in Japanese Patent Application Publication No. 2000-170905 (JP-A-2000-170905) and Japanese Patent Application Publication No. 2004-129450 (JP-A-2004-129450).
In the technique described in JP-A-2000-170905, a break/short-circuit fault is determined from a current flowing in the winding of each phase of an electric motor that is a drive source of a shift switching device, and upon determining a fault, the driver is notified of the fault by the lighting of an alarm lamp or the like. In the technique described in JP-A-2004-129450, a break detection circuit made up of two resistors is provided in the current supply line of the winding of each phase of an electric motor, thereby detecting a break in the winding of each phase on a phase-by-phase basis.
In a shift-by-wire type shift switching device, if a ground short-circuit occurs in one phase of an electric motor (for example, a three phase AC motor), the faulty phase remains energized, and since the faulty phase drags the other two phases, it is impossible to start up the rotation of the rotor of the electric motor. In contrast, in the case of a break fault, there are situations where it is possible to start up the rotation of the rotor. That is, if a break occurs in one of the three phases, current cannot be supplied to the winding of the phase in which the break has occurred, but the rotor may still be rotated by supplying current to the windings of the other two phases. Thus, if there is a break fault, there are situations where it is possible to continue switching of the shift range by rotating the electric motor, and it is desired to realize this.
However, since the fault detecting means of the related art (including the techniques described in JP-A-2000-170905 and JP-A-2004-129450) cannot discriminate between a break fault and a short-circuit fault, switching of the shift range by motor drive is not carried out even in the event of a break fault. That is, if current is supplied to the drive coil of an electric motor when it is impossible to discriminate between a break fault and a short-circuit fault and the motor fault is a short-circuit fault, there is a fear of an over-current flowing to the current supply line in the faulty phase. To avoid this, if a break/short-circuit fault occurs, the supply of current to the drive coil is stopped regardless of whether the fault is a break fault or a short-circuit fault.
Although it is possible to provide a short-circuit detection sensor to allow discrimination between a break and a short-circuit, in this case, the number of parts increases, which disadvantageously leads to an increase in cost. Also, it is not easy to realize a system that detects a ground short-circuit by using a sensor.